xxvi THE ANIMAL KINGDOM. 



and insects, the nervous system is fundamentally built nearly upon the same plan. 

 There is a pair of ganglia above the oesophagus, called the ' brain ; ' on the under side is 

 usually a second pair; the four, with the nei-ves or commissures connecting them, foi-m- 

 ing a ring. This arrangement of ganglia, often called the ' oesophageal ring,' consti- 

 tutes, with the slender nerve-threads leading away from them, the nervous system of 

 the lower worms, in many of which, however, as also in rnost Polyzoa and Brachio- 

 poda, the suboesophageal ganglia are wanting. Now to the oesophageal ring with its 

 two pairs of ganglia, add a third pair, the visceral ganglia, and we have the nervous 

 system of the clam and many molluscs. 



In the higher ringed worms, the Annulata, and in the Crustacea and Insects, there 

 is a chain of ganglia, or brains, which, behind the throat, are ventral, and lie on the 

 floor of the coelom or body cavity. The highest form of nerve-centre found in the 

 invertebrate animals, and which hints at the brain and skull of vertebrates, is the 

 mass of ganglia partly enclosed in an imperfect cartilaginous capsule in the head of 

 the cephalopods. The nervous cord of the Appendicularia, an ascidian, is con- 

 structed on the same plan as in the Annulats, but the mode of origin and apparently 

 dorsal position of the nervous system of the tailed larval ascidian presents features 

 which apparently anticipate the state of things existing among the lower vertebrates, 

 such as the lancelet. 



We need not here describe the different forms of nervous system in the classes of 

 invertebrates, but refer the reader to the figures and descriptions of the different 

 types in the body of this work. It will be well to read the following data concerning 

 the brain and nervous system, which we quote from Bastian's " The Brain as an Organ 

 of Mind." 



"1. Sedentary animals, though they may possess a nervous system, are often head- 

 less, and they then have no distinct morphological section of this system answering 

 to what is known as a brain. 



" 2. When a brain exists, it is invariably a double organ. Its two halves may be 

 separated from one another, though at other times they ai-e fused into what appears 

 to be a single mass. 



" 3. The component or elementary parts of the brain in these lower animals are 

 ganglia in connection with nerves proceeding from special impressible parts or sense- 

 organs ; and it is through the intervention of these united sensory ganglia that the 

 animal's actions are brought into harmony with its environment or medium. 



"4. That the sensory ganglia, which in the aggregate constitute the brain of 

 invertebrate animals, are connected with one another on the same side, and also with 

 their fellows on the opposite sides of the body. They are related to one another 

 either by what appears to be continuous growth, or by means of ' commissures.' 



" 5. The size of the brain as a whole, or of its several parts, is therefore always 

 fairly proportionate to the development of the animal's special sense-organs. The 

 more any one of these impressible surfaces or organs becomes elaborated and attuned 

 to take part in discriminating between varied external impressions, the greater will be 

 the proportionate size of the ganglionic mass concerned. 



" 6. Of the several sense-organs and sensory ganglia whose activity lies at the root 

 of the instinctive and intelligent life (such as it is) of invertebrate animals, some are 

 much more important than others. Two of them especially are notable for their 

 greater proportional development, viz. : those concerned with touch and vision. The 

 organs of the former sense are, however, soon outstripped in importance by the latter. 



